Protected nano-particle assemblies

ABSTRACT

A spectroscopically active nano-particle assembly is provided. The nano-particle assembly includes a cluster of metallic nano-particles. A first protective coating is formed over a first side of the cluster, and a second protective coating is formed over a second side of the cluster, wherein the second side of the cluster is opposite the first side.

BACKGROUND

Spectroscopic tagging may use emissive particles that are printed, attached, or react with the target. These may be used for identification purposes, tracking purposes, in-vivo analyses, and the like. For example, emissive particles based on Surface Enhanced Raman Spectroscopy (SERS) may be used for tagging in biological systems, such as reacting with bacteria, proteins, DNA, and the like, to allow the determination of presence and concentration. However, typical solution-based biotagging based on SERS may suffer from weak signal, low contrast, and poor reproducibility due to poor particle shape, size and assembly control. While individual nano-particles are the simplest to produce, they may also have the lowest Raman enhancement. Higher response may be provided by agglomerations of particles, but these depend on random processes which have poor control and low yield.

DESCRIPTION OF THE DRAWINGS

Certain exemplary embodiments are described in the following detailed description and in reference to the drawings.

FIG. 1 is a drawing of an example of a substrate supporting a column layer for the formation of flexible nanopillars.

FIG. 2 is a drawing of an example of a group of flexible nanopillars formed on the substrate.

FIG. 3 is a drawing of an example of a metal layer deposited over the flexible nanopillars and the substrate, forming metal caps at the top of each of the flexible nanopillars.

FIG. 4 is a schematic drawing of an example of the evaporation of a fluid.

FIG. 5 is a drawing of an example of collapsed groups formed by the evaporation of the fluid.

FIG. 6 is a drawing of an example of coating the substrate including the collapsed groups with a layer that forms a protective coating over the collapsed groups.

FIG. 7 is a drawing of an example of imprinting collapsed groups that have been coated with the protective coating into a polymeric film.

FIG. 8 is a drawing of an example of the collapsed groups pressed into the polymeric film.

FIG. 9 is a drawing of an example of the substrate holding the flexible nanopillars after removal from the polymeric film.

FIG. 10 is a drawing of an example of the application of a second protective coating over the polymeric film and the embedded metal caps.

FIG. 11 is a drawing of an example of the nano-particle assemblies suspended in a solution, for example, after the polymeric film is dissolved.

FIG. 12 is a process flow diagram of an example of a method for forming nano-particle assemblies.

FIG. 13 is a schematic diagram of an example of a spectroscopic analysis using nano-particle assemblies that are suspended in a solution.

DETAILED DESCRIPTION

A sensing or tagging system based on engineered and encapsulated three-dimensional (3-D) nano-particle assemblies is described herein. Although, the examples described focus on Raman spectroscopy, it may be understood that the nano-particle assemblies be used with any number of other spectroscopic analyses including, for example, surface enhanced luminescence, fluorimetry, and the like.

Raman based tagging systems often consist of Raman reporter molecules trapped in the “hot spot” of a plasmonic nano-particle assembly, which may be an individual particle, or a dimer, trimer, tetramer, or pentamer. When the hot spot is excited, for example, by laser light of the appropriate wavelength, the reporter molecules emits a signature radiation pattern, which is enhanced due to its location in the hot spot. As used herein, this technique is referred to as Surface Enhanced Raman Spectroscopy (SERS).

When the nanoparticle assemblies are functionalized to bind to specific regions, such as tissues or cells in the human body, the emitted radiation may be used to map such regions. Engineered particle assemblies may provide stronger and more uniform SERS enhancement than random agglomeration. Techniques have been developed to fabricate 2-D assemblies on a substrate. The techniques described herein allow the formation of 3-D particles that include Raman reporter molecules. As noted herein, spectroscopically active molecules for other techniques may be used in addition to, or instead of, the Raman reporters. The particle assemblies are encapsulated with a protective coating, such as silicon dioxide, which may protect the Raman reporter molecule and help stabilize the particles in solution, for example, promoting the stability of a suspension, blocking further aggregation, and the like.

The nanoparticles assemblies may be tuned to specific applications. For example, the optimal size and geometry of the nanoparticle assemblies may be different for applications where the Raman tags bind to a substrate or tissue, need to pass through cell walls, or are used in solution, among others. The encapsulation method allows functionalization for targeting, for example, by reacting the protective coating with a targeting molecule.

FIG. 1 is a drawing of an example of a substrate 102 supporting a column layer 104 for the formation of flexible nanopillars. The substrate 102 may be made from silicon, glass, quartz, silicon nitride, sapphire, aluminum oxide, diamond, diamond-like carbon, or other rigid inorganic materials, such as metals and metallic alloys. In some examples, the substrate 102 may be a polymeric material, such as a polyacrylate, a polyamide, a polyolefin, such as polyethylene, polypropylene, or a cyclic olefin, a polycarbonate, polyesters, such as polyethylene terephthalate, polyethylene napthalate, or other polymeric material suitable for making films. Any of these polymeric materials may be a copolymer, a homopolymer, or combination thereof. The substrate 102 may be a web used in a roll-to-roll fabrication process.

The column layer 104 may be a polymeric material that can be formed into columns by any number of processes, such as described with respect to FIG. 2. Polymeric materials that may be used include but are not limited to, photo resists, hard mold resins such as PMMA, soft mold polymers such as PDMS, ETFE or PTFE, hybrid-mold cross-linked, uv-curable or thermal-curable, polymers based on acrylate, methacrylate, vinyl, epoxy, silane, peroxide, urethane or isocyanate. The polymer materials may be modified to improve imprint and mechanical properties with copolymers, additives, fillers, modifiers, photoinitiators, and the like. Any of the materials mentioned with respect to the substrate 102 may also be used. A column layer 104 does not have to be used to form the nanopillars. In some examples, the substrate 102 may form the column layer 104, while in other examples, the nanopillars may be directly formed on the substrate 102.

FIG. 2 is a drawing of an example of a group 202 of flexible nanopillars 204 formed on the substrate 102. Like numbered items are as described with respect to FIG. 1. The flexible nanopillars 204 may be formed from a column layer 104 (as described with respect to FIG. 1) on the surface of the substrate 102 by any number processes, including nano-embossing, lithography followed by reactive ion etching or chemical etching, UV or thermal curing, and the like. In a nano-embossing process, a column layer 104 may be softened and then run through a die to form the flexible nanopillars 204 from the column layer 104. Any number of other processes known in the art may be used to form the flexible nanopillars 204 from a column layer 104. Further, the column layer 104 may be part of the substrate 102, wherein lithographic patterning and etching techniques may be used.

In some examples, the flexible nanopillars 204 may be deposited on the substrate 102, for example, using nano-printing, ion deposition techniques, and the like. In a nano-printing process, the materials forming the flexible nanopillars 204 may be directly deposited, or printed, on the surface of the substrate 102. In other examples, nano-wires may be grown on the substrate through ion deposition. In growing the nano-wires to produce the flexible column, nano-wire seeds may be deposited onto the substrate 102. In some examples, the nano-wire seeds may be silicon nano-structures, and the nano-wires may be silicon dioxide structures grown during chemical vapor deposition from silane. Once the flexible nanopillars 204 are formed, metal caps may be formed over the nanopillars.

FIG. 3 is a drawing of an example of a metal layer 302 deposited over the flexible nanopillars 204 and the substrate 102, forming metal caps 304 at the top of each of the flexible nanopillars 204. Like numbered items are as described with respect to FIGS. 1 and 2. The metal caps 304 may include noble metals, such as ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au), as well as copper, or alloys thereof. Other metals may be used in the metal caps 304, such as aluminum (Al), titanium (Ti), or other metals. The metal layer may be deposited using a thin-film vacuum-apparatus to deposit metal onto the flexible nanopillars 204. The metal may be deposited at an angle of about 30° to a surface of the substrate 102 to enhance formation of the metal caps 304, while decreasing the amount of metal deposited in other locations, such as in the metal layer 302 over the substrate 102. The material deposited from the metallic vapor may also be limited to control the deposition, and lower the amount deposited on the substrate 102 or on sides of the flexible nanopillars 204.

Other techniques may be used to form the metal caps 304. In some examples, the substrate 102 including the flexible nanopillars 204 may be immersed in a plating solution that includes metal cations. An electric potential applied to the substrate 102 may cause deposition of metal at the top of the flexible nanopillars 204, as the top of the flexible nanopillars 204 may have a more concentrated or enhanced electrical field. The metal caps 304 may be precipitated from colloidal suspensions of metallic nanoparticles when an electric potential is applied to the substrate 102. Any number of other techniques may be used to form the metal caps 304.

FIG. 4 is a schematic drawing of an example of the evaporation of a fluid 402. Like numbered items are as described with respect to FIGS. 1, 2, and 3. The fluid 402 may include reporter molecules 404. The report molecules 404 may be any number of molecules that give a spectroscopic response, for example, in a surface enhanced luminescence technique. For use in SERS, the reporter molecule may be trans-1,2-bis (4-pyridyl) ethylene.

Evaporation of the fluid 402 provides increased microcapillary pressure around the flexible nanopillars 204. This causes flexible nanopillars 204 to collapse, bringing the metal caps 304 of adjacent flexible nanopillars 204 together, forming a cluster of metallic nano-particles attached to the flexible stalks of the flexible nanopillars.

However, any number of other reporter molecules may be used depending on the spectroscopic techniques. In some examples, the fluid 402 with the reporter molecules 404 may be incubated before the evaporation to allow reactions before the evaporation.

Multiple fluids may be used in the preparation. For example, the reporter molecules 404 may be dissolved in a first fluid and the flexible nanopillars 204 and metal caps 304 may be incubated to allow reaction. A clean fluid may be used to rinse the surface to eliminate excess reporter molecules 404. The same fluid or another fluid may then be evaporated from the surface to collapse the nanopillars 204.

Reporter molecules 404 do not have to be added by dissolution in the fluid 402. In some examples, the metal caps 304 of the flexible pillars 204 may be reacted with gas phase reporter molecules after which a fluid 402 may be used to collapse the flexible nanopillars 402.

FIG. 5 is a drawing of an example of collapsed groups 502 formed by the evaporation of the fluid. Like numbered items are as described with respect to FIGS. 1 to 4. The reporter molecules 404 may be adsorbed on the surfaces of the metal caps 304, or may be trapped in the nm-scale gaps between the metal caps 304. The number of metal caps 304 in each collapsed group 502 may vary depending on the dynamics of the collapse process. For example, a collapsed group 502 may include a cluster of metallic nano-particles formed from two metal caps 304, three metal caps 304, four metal caps 304, or five metal caps 304, or more. The pentamer shape, formed from five metal caps 304, has been shown to have particularly good performance on a substrate and may also perform well in solution due to the large number of dipole axes.

FIG. 6 is a drawing of an example of coating the substrate including the collapsed groups 502 with a layer 602 that forms a protective coating 604 over the collapsed groups 502. Like numbered items are as described with respect to FIGS. 1, 3, and 5. As described herein, the protective coating 604 may be a layer of silicon dioxide (SiO₂). Other materials may be used to form the protective coating 604, such as metal oxides, metal carbides, metal sulfides, diamond, spinel structures (MgAl₂O₄), and the like. The protective coating 604 may be applied using a vapor deposition technique, atomic layer deposition, or liquid deposition techniques. The protective coating 604 forms protected collapsed groups.

FIG. 7 is a drawing of an example of imprinting collapsed groups 502 that have been coated with the protective coating 604 into a polymeric film 702. Like numbered items are as described with respect to FIGS. 1, 3, 5, and 6. The polymeric film 702 may be layer of polymethylmethacrylate (PMMA) that is heated above the glass transition (Tg) temperature. This softens the polymeric film 702 to allow penetration of the surface by the protective coating 604. Any number of other materials may be used to form the polymeric film 702, like polyolefins such as polystyrene, polycarbonate, polyesters such as polyethylene terephthalate, among others.

FIG. 8 is a drawing of an example of the collapsed groups 502 pressed into the polymeric film 702. Like numbered items are as described with respect to FIGS. 1, 3, 5, 6, and 7. The collapsed groups 502 are left in place while the polymeric film 702 cools and hardens. Once the polymeric film 702 has hardened, the substrate 102 may be removed as described with respect to FIG. 9.

FIG. 9 is a drawing of an example of the substrate 102 holding the flexible nanopillars 204 after removal from the polymeric film 702. Like numbered items are as described with respect to FIGS. 1, 2, 3, 6, and 7. The removal of the substrate 102 may break or disconnect the flexible nanopillars 204 from the metal caps 304, leaving the metal caps 304, covered by the protective coating 604, embedded in the polymeric film 702.

FIG. 10 is a drawing of an example of the application of a second protective coating 1002 over the polymeric film 702 and the embedded metal caps 304. Like numbered items are as described with respect to FIGS. 3, 6, and 7. The application of the second protective coating 1002 forms nano-particle assemblies 1004 that may be used for spectroscopic tagging, as described herein. Once the second protective coating 1002 is formed, the nano-particle assemblies 1004 may be isolated. The vertical quality of the deposition by evaporation, and the rounded corners of the particles, leave a small uncoated gap, or exposed portion, between the metal caps 304 and the polymeric film 702. This allows a solvent to release the assemblies into solution by dissolving the polymeric film 702.

FIG. 11 is a drawing of an example of the nano-particle assemblies 1004 suspended in a solution, for example, after the polymeric film is dissolved. Like numbered items are as described with respect to FIGS. 3, 6, and 10. In order to simplify FIG. 11, not every metal cap 304, protective coating 604 and 1002, or exposed portion 1102 is labeled. The nano-particle assemblies 1004 may be isolated from a solution, and may then be suspended in a reagent solution as shown in FIG. 11, for example, for further preparations, such as functionalization.

The nano-particle assemblies 1004 may be functionalized using molecules with a binding agent on one end and a functional group on the other end. For example, the binding agent may include thiols for binding to the exposed portion 1102 of metal caps 304 that include gold, or silanes for binding to the protective coating 604 or 1002.

The functional group can be designed to promote stability in solution, bind to specific substrates or tissues in a body, or both, if multiple functionalization molecules are used. An example of a functional group may be a monoclonal antibody selected to bind to and expressed group on the surface of a cell. Further, a functional group may include a short DNA segment configured to bond to a complementary DNA segment. As another example, the functional group may be a long paraffinic chain to stabilize the nano-particle assemblies 1004 in organic materials, such as solvents, fatty acids, triglycerides, and the like.

In some examples, the functional group may be a therapeutic agent, for example, that is released when the nano-particle assemblies 1004 are exposed to an excitation source. This provides a method for delivering local therapeutics with a plasmonically activated drug delivery system using nano-particles assemblies 1004.

FIG. 12 is a process flow diagram of an example of a method 1200 for forming nano-particle assemblies. The method 1200 begins at block 1202 when flexible columnar structures, such as flexible nanopillars, are formed over a substrate. This may be performed, for example, as described with respect to FIG. 2.

At block 1204, a metal coating is formed over the flexible columnar structures, for example, as described with respect to FIG. 3. The metal coating forms a cap over top surface of each of the flexible columnar structures.

At block 1206, a fluid is placed over the flexible columnar structures. The fluid may include a reporter molecule, as described with respect to FIG. 4. At block 1208 the fluid is allowed to evaporate, pulling the flexible columnar structures together into collapsed groups. Each collapsed group includes at least two flexible columnar structures, and may include three, four, or five, or more flexible columnar structures.

At block 1210, a protective coating is formed over the collapsed groups, for example, as described with respect to FIG. 6. At block 1212, the collapsed groups are pressed into a softened film with the protective coating pointing into the softened film, for example, as described with respect to FIGS. 7 and 8. The film is then allowed to harden.

At block 1214, once the film has hardened, the flexible columnar structures are removed, leaving the metal groups in the film. At block 1216, the film is coated with a second protective coating, forming the nano-particle assemblies. This protective coating may be the same or different than the first protective coating.

At block 1218, the film may be dissolved to release the nano-particle assemblies into a solution, for example, as described with respect to FIG. 11. The nano-particle assemblies may then be isolated from the solution used to dissolve the film, and may then be re-suspended in a new solution. Reagents may be added to the new solution to functionalize the nano-particle assemblies, for example, to allow the nano-particle assemblies to attach to target structures, be stabilized in solution, or both.

FIG. 13 is a schematic diagram of an example of a spectroscopic analysis 1300 using nano-particle assemblies 1004 that are suspended in a solution 1306. Like numbered items are as described with respect to FIG. 10. The nano-particle assemblies 1004 may be associated with target items, such as cells, compounds, and the like. Further, the nano-particle assemblies 1004 may be used in any number of in-vitro, or in-vivo applications, such as mapping cancer cells in a tissue sample, or identifying an item in a printed label, among many others. For example, the nano-particle assemblies 1004 may be injected into a living organism to perform a spectroscopic analysis within the living organism, such as mapping locations of particular types of cells, among others.

In this example, the nano-particle assemblies 1004 are being probed by an excitation beam 1302 of electromagnetic radiation, for example, from a laser or a spectrophotometer or fluorimeter. The excitation beam 1302 interacts with active surfaces in the nano-particle assemblies 1004 that may hold reporter molecules adsorbed by collapsed groups.

In response to the excitation beam 1302, electromagnetic radiation may be emitted from the active surfaces in the nano-particle assemblies 1004. The location, or intensity, of the emitted radiation 1304 may depend on the associated target, for example, being more intense in regions where the target is located, or where the nano-particle assemblies 1004 are printed, among others. This may provide information that can be used to map or identify the target. The metal caps of the collapsed groups provide a plasmon resonance that may interact with the analyte species enhancing the spectroscopic response of the analyte species.

The excitation beam 1302 and the emitted radiation 1304 may be at wavelength ranges extending from the near ultraviolet to the near infrared. For example, this may cover a wavelength range from about 150 nanometers (nm) to about 2,500 nm. Accordingly, the nano-particle assemblies 1004 may be used for mapping targets using surface enhanced spectroscopy (SES), such as surface enhanced Raman spectroscopy (SERS), or other surface enhanced luminescence (SEL) techniques, such as fluorimetry.

As described herein, lithographic patterning of relatively large features, such as the nanopillars or other flexible columnar structures, followed by guided self-assembly during the solvent evaporation may produce higher-quality groups that provide a stronger spectral response than other techniques. This may allow the use of a lower concentration of the nano-particle assemblies 1004, allowing for transparent solutions, and less contamination of targets. This may be particularly useful for in vivo applications. The encapsulation promotes stability, providing a more uniform response, and may also be used for functionalization.

While the present techniques may be susceptible to various modifications and alternative forms, the exemplary examples discussed above have been shown only by way of example. It is to be understood that the technique is not intended to be limited to the particular examples disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the scope of the present techniques. 

What is claimed is:
 1. A spectroscopically active nano-particle assembly, comprising: a cluster comprising metallic nano-particles; a first protective coating formed over a first side of the cluster; and a second protective coating formed over a second side of the cluster, wherein the second side of the cluster is opposite the first side.
 2. The spectroscopically active nano-particle assembly of claim 1, wherein the cluster of metallic nano-particles comprise gold, silver, or both.
 3. The spectroscopically active nano-particle assembly of claim 1, wherein the cluster of metallic nano-particles comprise aluminum, copper, or a noble metal, or any combinations thereof.
 4. The spectroscopically active nano-particle assembly of claim 1, wherein the cluster comprises five metallic nano-particles.
 5. The spectroscopically active nano-particle assembly of claim 1, wherein the cluster of metallic nano-particles comprises reporter molecules.
 6. The spectroscopically active nano-particle assembly of claim 1, wherein the first protective coating, the second protective coating, or both, comprises silicon dioxide, metal oxides, metal nitrides, or metal carbides, or any combinations thereof.
 7. A method for using a spectroscopically active nano-particle assembly, comprising: forming a first protective coating over a plurality of collapsed groups to form protected collapsed groups, wherein each collapsed group comprises a cluster of metal caps in proximity to each other, and wherein each metal cap is at a top of a flexible stalk; pressing the plurality of protected collapsed groups into a film layer that has been softened by heating; removing each of the flexible stalks, leaving metal clusters embedded in the film layer; cooling the film layer to harden the film layer; forming a second protective coating over the film layer and the metal clusters embedded in the film layer; and dissolving the film layer to free the metal clusters.
 8. The method of claim 7, comprising functionalizing the first protective coating, the second protective coating, or both.
 9. The method of claim 7, comprising: injecting the spectroscopically active nano-particle assembly into a living organism; and performing a spectroscopic analysis on the spectroscopically active nano-particle assembly in the living organism.
 10. The method of claim 7, comprising: forming a plurality of flexible columnar structures on a substrate; forming a metal coating over the plurality of flexible columnar structures, wherein the metal coating forms a cap over a top surface of each of the plurality of flexible columnar structures; placing a fluid over the plurality of flexible columnar structures; and evaporating the fluid, wherein evaporation of the fluid exerts a microcapillary pressure that pulls the plurality of flexible columnar structures together into the plurality of collapsed groups, wherein each collapsed group comprises at least two flexible columnar structures.
 11. The method of claim 10, comprising dissolving a reporter molecule into the fluid, wherein the reporter molecule is trapped between two adjoining metal caps during the evaporation.
 12. The method of claim 10, comprising forming collapsed groups comprising a pentamer of metal caps.
 13. An analysis solution, comprising a suspension of spectroscopically active nano-particle assemblies, wherein each of spectroscopically active nano-particle assembly comprises: a cluster comprising metallic nano-particles; a first protective coating formed over a first side of the cluster; and a second protective coating formed over a second side of the cluster, wherein the second side of the cluster is opposite the first side.
 14. The analysis solution of claim 13, wherein the spectroscopically active nano-particle assemblies are configured to be used in a surface enhanced Raman spectroscopy analysis.
 15. The analysis solution of claim 13, wherein the first protective coating, the second protective coating, or both, comprise a functional group to promote stability in the analysis solution, bind to a specific substrate, or both. 